This invention relates to sensory prostheses, and in particular, to a prosthesis for ameliorating the symptoms of balance or vestibular disorders.
In order to stand erect, one constantly makes minor adjustments to avoid falling over. These minor adjustments involve the often subtle contraction and relaxation of a large number muscles. The choice of which muscles to contract and how hard to contract them depends on one""s perception of spatial orientation. The vestibular system contributes substantially to one""s perception of spatial orientation.
The standing human being is thus a feedback control system in which the vestibular system provides motion signals to the brain. Without these motion signals, the feedback control system rapidly becomes unstable. This instability is manifested in general clumsiness, frequent collisions, and spontaneous falls. Among the elderly, such falls account for over half of all accidental deaths.
Persons with improperly functioning balance systems can also experience difficulty compensating for head motion when attempting to gaze in a particular direction. This results in such symptoms as blurred vision while walking, or the inability to read when in a moving vehicle.
A person whose lack of vestibular function affects primarily balance can compensate by using a cane or walker. However, these devices require the use of one or both hands. In addition, these devices do not attempt to replace the sense of balance. Instead, they merely reduce the likelihood of injury that arises from loss of one""s sense of balance.
At present, a person whose lack of vestibular function results in an inability to gaze in a selected direction has little choice but to endure the condition.
The invention provides a balance prosthesis that is small enough and light enough to be worn in the course of one""s daily activities. Such a balance prosthesis provides the wearer information indicative of the spatial orientation of a body part (most commonly the head) of the wearer.
As used herein, the term xe2x80x9cwearerxe2x80x9d refers to any animal that wears the balance prosthesis and receives information therefore. The wearer can be a human under a doctor""s care or any other animal. The wearer need not have a balance disorder. For example, the wearer of the balance prosthesis can be a human or animal research subject. Alternatively, the wearer can be a human or animal seeking sensory enhancement provided by the balance prosthesis.
The balance prosthesis includes a motion sensing system to be worn on the body part whose orientation is sought. The motion sensing system generates a motion signal indicative of motion experienced by that body part. The motion signal is then passed to a signal processor, which generates an estimate of the spatial orientation of the body part on the basis of the motion signal.
The balance prosthesis also includes an encoder in communication with the signal processor. The encoder generates a feedback signal on the basis of the estimate of the spatial orientation. This feedback signal is provided to a stimulator configured to provide a signal to the nervous system in response to the feedback signal.
In one embodiment, the stimulator includes a tactor set that includes one or more tactors adapted to be worn against skin of the wearer. The feedback signal causes one or more of these tactors to vibrate, in response to a control signal from the encoder.
A variety of modulation methods are available to the encoder for communicating spatial orientation to the wearer. In one aspect of the invention, the encoder pulses a selected tactor at a pulse repetition frequency indicative of the spatial orientation. In another aspect of the invention, the encoder is configured to select a subset of the tactor set on the basis of spatial orientation and to excite only the tactors in that subset. In yet another aspect of the invention, the encoder is configured to sequentially excite a plurality of subsets of the tactor set according to an excitation sequence. The plurality of subsets and the excitation sequence are selected on the basis of the spatial orientation.
In another embodiment of the invention, the stimulator is an electrode in communication with the encoder. The electrode is adapted to be inserted proximate to a nerve in the wearer and to carry a signal indicative of the spatial orientation. Again, a variety of modulation methods are available for communicating spatial orientation to the wearer. In one aspect of the invention, the encoder is configured to apply a sequence of pulses to the electrode at a pulse repetition frequency indicative of the spatial orientation. In another aspect of the invention, the encoder is configured to energize the electrode with an energizing amplitude selected on the basis of the spatial orientation.
In another embodiment, the encoder provides a feedback signal that depends not only on the spatial orientation of the wearer""s body part but also on the activity in the wearer""s nervous system, This embodiment includes a measurement electrode in communication with the encoder and adapted for placement in communication with a nerve. The measurement electrode detects an endogenous signal in the nerve and provides the information in that endogenous signal to the encoder. The encoder then generates a feedback signal on the basis of the both the information contained in the endogenous signal and the estimate of the spatial orientation provided by the digital signal processor.
Other stimulators can be substituted or used in addition to those described above. For example, the stimulator can provide an acoustic signal to the wearer. Alternatively, the stimulator can provide a visual signal. Such a stimulator can be incorporated into a pair of glasses.
One or more additional signals can be provided to the encoder for use in generating a feedback signal. For example, in some cases, it is useful to provide a feedback signal that depends not only on the spatial orientation of one body part but on the spatial orientation of additional body parts of the wearer. In this embodiment, the balance prosthesis includes an additional motion sensing system to be worn on an additional body part of the wearer. The additional motion sensing system generates an additional motion signal indicative of motion experienced by the additional body part of the wearer. The balance prosthesis also includes an additional signal processor in communication with the additional motion sensing system and the encoder. The additional signal processor is configured to generate an estimate of the spatial orientation of the additional body part on the basis of the additional motion signal and to provide that estimate to the encoder.
Other types of additional signals can be provided to the encoder for use in generating a feedback signal. For example, the additional signal can be one that indicates the location of the wearer relative to an external coordinate system. The additional signal can also indicate other properties associated with one or more body parts, such as joint angles of various joints. The additional signal can also provide information indicative of tactile pressure experienced by the wearer.
The motion sensing system can include at least one rotation sensor. Typically, the rotation sensor is a micro-mechanical device having a proof mass that undergoes a periodic motion susceptible to disturbance by a rotation. An example of such a micro-mechanical device is a tuning fork gyroscope.
The motion sensing system can also include at least one translation sensor. Typically, the translation sensor is a micro-mechanical device having a proof mass. The proof mass has a position that deviates from an equilibrium position in response to a linear acceleration.
In one embodiment of the balance prosthesis, the signal processing system includes a low-pass filter to filter a first inertial guidance signal and a high-pass filter to filter a second inertial guidance signal. The low-pass filter and the high-pass filter have complementary filter transfer functions.
In another embodiment of the balance prosthesis, the signal processing system is configured to distinguish linear acceleration due to gravity from linear acceleration resulting in translation. The signal processing system can achieve this by determining a current direction of a gravity vector on the basis of an integrated rotation sensor output. Having determined this direction, the signal processing system can then remove the effect of the gravity vector from a translation sensor output.
These and other features and advantages of the invention will be apparent from the following detailed description, and the figures, in which: